AR mag question

I have an AR-15 High Standard M-4 and I am having a problem seating a new magazine when the bolt is closed. It will not seat and lock in the closed position. If I open the bolt everything works fine. Is this normal or do I have a problem?

I have a couple differnt brands and have run into a few problems with ones from C Products and even Colt mags. I have a couple AR Stoner mags that have been very good for me. Others here may be able to give some others that they've had good luck with.

Its supposed to be tough with the bolt closed, especially with a loaded mag. essentially you are forcing the compressed spring about another 1/8th to 1/4 inch down before the mag locks into place. I always lock the bolt open, insert mag and drop the bolt with the release because sometimes i will put a loaded mag in and charge the rifle to have the mag fall out because it didnt lock into place and i thought it did... I use magpul p-mag 20 rd mags. they are the best in the west IMO

I personally have fallen quite in love with PMAGs, as have all my guys. On the upside, they're getting much more inexpensive as time marches on. I used to be quite weary of polymer mags, but Magpul PMAGs have stood up to everything we've put em thru, so I'll back em all day.